Solar Power: How Many Panels Do I Need?

Right-Sizing Your Solar Power System

One of the most poplar questions I'm asked is "How Many Panels Do I Need?". 

I suppose they expect me to give them a specific number, but the real answer is a bit more complex, but not that hard to understand. The answer that I always give is "How much power do you want?"

It's interesting that when I give them that answer they usually seem a bit perplexed. The reason is that with commercial power that we're all used to, we can have all the power we want if we can pay the monthly bill. So most of don't even know how much power we use, we just pay it...

Modern homes are usually built without regard to electrical energy savings. Construction codes are based on important safety issues but are not focused on energy consumption. Specific appliances like furnaces can be extremely efficient, and new homes built nowadays are often very well insulated but we seldom have any idea as to how much electricity our home uses.

The first step is to assess your needs. To get the right answer on the number of panels and what size they need to know how much power you need. If you are on commercial power now get out your utility bill and look closely at how many KWH you use monthly throughout the year.

Many people I've met use 1000KWH or electricity or more a month. Trying to go completely solar is going to be expensive if your utility bill is this high but it's still very possible. Especially now, when solar panels are so much cheaper than they were a few years ago.

By reducing the power we need from the sun you can save a lot of money on your system. Conserving on electrical energy is easy to do and should be an important part of building your solar power system. New technology often means we can save energy without giving up comfort and quality of life.

If you want to test appliances to see how much power you are using, you can use an AIMS "Kill A Watt" meter that will allow you see how much power your refrigerator (or any other 110v appliance) uses each day. I have used this device a lot and I've been very surprised at what I discovered.  The more you can know about your usage of electricity, the better.

Don't be optimistic planning your system. Be sure you are looking at things with a realistic or even slightly pessimistic view. If you have a few extra panels fine, but you don't have enough you'll be unhappy with the results, and while you will be collecting power it won't feel like you've accomplished much.

The analogy I usually use goes like this: Suppose you need 10 gallons of fuel to get to town, but all you can get is 5 gallons. It might be nice that you only have to walk half of the way, but even if the fuel is free the lack of what you need and the long walk is what you're going to remember! Electricity is critical for daily life, so don't cut your estimate short.

You need to do a simple site assessment. Weather is an obviously critical factor with solar power. "Solar Insolation" maps are available for the US that can help you estimate what the power yield will be for your panels. These maps take a variety of factors into account and are an important resource.

If you live in an area where sunshine is limited or where trees an other obstacles block the sun from your panels you can compensate by adding more, but only to a degree. In some cases solar power is not a sensible investment because the number of panels required is just too great due to site considerations.

Your panels need to be facing south. If you have a south facing roof on your home, this could be an ideal place to mount your solar array. If not, various mounting systems are available which give you a lot of interesting options.

Once we know how much power we need the rest is simple math.

In our case we have an average of 4.2 hours per day of sun according to our Solar Insolation map. If I buy a professional solar panel and the rated power of that panel is 250Watts, I can collect about 1,000 watts (1KWH) each day from that panel. In my case I need 450KWH per month. That works out to be 15KWH per day, so I need 15 250 watt panels at the minimum. The more panels I have, the more margin I have to compensate for bad weather. In my case I want 30 panels, so that my battery system can quickly recover from bad weather, and if the weather goes bad in the afternoon, I've already collected the power that I need in the morning.

You now know how to calculate the number of solar panels needed, but there are a list of things you need to figure out to complete your system.

  • If you are going to live off the grid you need a properly sized battery bank
  • You need a generator, for serious bad weather backup
  • You need a very reliable commercial-grade inverter to convert battery power to AC
  • You need mounts to hold your panels
  • You need PPT charging regulators to take the power from your solar panels and charge your batteries.

I've been living off the grid for nearly 20 years, and it has been fun. I've made a lot of mistakes and done things that have really worked out well. I like writing about my experiences to help others build reliable, quiet, independent lifestyles from the sun.

There has been many occaisions where our neighbors are without power for hours and we didn't even know there was an outage. Our power has been more reliable then I ever would have thought. It feels great to not have a utility bill each month!

Once your properly designed system is installed it just works.

Amazon Price: Buy Now
(price as of Aug 2, 2016)
This is a very reliable inverter that we use here.
Find out how much power your appliances use!